High strength polymeric materials are being increasingly used to replace traditional structural materials, such as metal, in many applications. The polymeric materials have the advantage of lower weight and are often less expensive and more durable than metals. However, polymeric materials tend to be much lower in strength than metal. Unless polymeric materials are reinforced in some manner, they often do not meet the strength requirements for metal replacement.
Thus, polymeric composites have been developed to meet such strength requirements. These composites are characterized by having a continuous polymeric matrix within which is embedded a reinforcement material, which is usually a relatively rigid, high aspect ratio material such as glass fibers.
Such composites are typically molded into a predetermined shape, which is in many cases asymmetric. To place the reinforcement material into the composite, the reinforcement material is usually placed into the mold in a first step, followed by closing the mold and then introducing a fluid molding resin. The molding resin fills the mold, including the interstices between the fibers, and hardens (by cooling or curing) to form the desired composite. Alternatively, the molding resin can be applied to the reinforcing fiber prior to molding. The reinforcing fiber with resin thereon is then placed into a mold where temperature and pressure are applied, curing the resin to prepare the desired composite.
It is desirable to uniformly distribute the reinforcement material throughout the composite. Otherwise, the composite will have weak spots where the reinforcement is lacking. Thus, it is important to prepare the reinforcement material so that the individual fibers are distributed evenly throughout the composite. In addition, the individual fibers should be held in place to resist flowing with the molding resin as it enters the mold, which would disrupt the fiber distribution.
For these reasons, reinforcement has been conventionally formed into a mat outside of the mold. The preform mat is then placed in the mold and either impregnated with resin to make the final composite article, or simply heated and pressed to make a very low density composite article. The mat is generally prepared by forming the reinforcing fibers into a shape matching the inside of the mold and applying a binder to the fibers. In some instances, a thermosetting binder is pre-applied, and then cured after the fibers are shaped into a mat.
In other methods, a thermoplastic binder is applied, so that in a subsequent operation the binder can be heated and softened and the mat subsequently shaped. This binder “glues” the individual fibers to each other so that the resulting mat retains its shape when it is transferred to the mold for further processing. The binder also helps the individual fibers retain their positions when the fluid molding resin is introduced into the mold. In some cases, a molding resin can alternatively be applied to the reinforcing fiber prior to molding. The fiber with binder and resin is placed into a mold where temperature and pressure are then applied, curing the resin to prepare the desired composite.
Binders conventionally used have been primarily of three types, each of which have various drawbacks. The predominantly used binders have been solvent-borne polymers, i.e., liquids, such as epoxy and polyester resins. The solvent-borne binders are usually sprayed onto the mat via an “air-directed” method, and then the mat is heated to volatilize the solvent and, if necessary, cure the binder. This means that the application of binder is at least a two-step process, which is not desirable from an economic standpoint. Also, the use of solvents is encountered, which raises environmental, exposure and recovery issues. Dealing with these issues potentially adds significantly to the expense of the process. The procedure is also energy intensive, as the entire mat must be heated just to flash off solvent and cure the binder. The curing step also makes the process take longer.
Use of the solvent-borne polymer binders is extremely messy. There are also high maintenance costs associated with keeping the work area and the screen on which the mat is formed clean. In this case, where the binder may be low viscosity fluid, it tends to flow over and coat a large portion of the surface of the fibers. When a composite article is then prepared from a preform made in this way, the binder often interferes with the adhesion between the fibers and the continuous polymer phase, to the detriment of the physical properties of the final composite.
A second form of binder is powdered binders. These can be mixed with the fibers, and then the mass formed into a preform shape, which is heated to cure the binder in situ. Alternatively, these binders can be sprayed to contact the fibers. However, simply substituting a powdered binder in an air-directed method raises problems. For example, powdered binders cannot be applied unless a veil is first applied to the screen to prevent the binder particles from being sucked through. Again, this adds to the overall cost and adds a step to the process. Airborne powders may also present a health and explosion hazard, depending on conditions of use. The use of powdered binders additionally requires a heating step to melt the binder particles after they are applied to the fibers. Heating renders this process energy-intensive.
Binders of a third type are heated thermoplastic materials, which can be melted and sprayed as a binder. Use of these materials makes any subsequent heating step unnecessary, since the binder does not require heat to achieve some undetermined measure of adhesion to the fibers. This method has problems with “lofting,” or inadequate compaction of the preform. Lofting typically occurs because the thermoplastics are conventionally heated to any random temperature above their melting points, leading to a lack of uniformity in their cooling patterns and extensive migration along fiber surfaces. This allows some of the fibers to “bounce back” before they are set into place by the solidifying thermoplastic. This may result in formation of a lower density preform than desired, density gradients throughout the preform, and poor adhesion of the fibers to each other.
In view of the problems discussed herein, one prior art method disclosed in U.S. Pat. No. 6,030,575, which is incorporated herein by reference, applies a heated binder to fibers already supported on a support surface while a vacuum is applied to the other side of the support surface. By this method, the fibers are held in place by the vacuum while the binder is applied at a high pressure by a spray device. This application applies pressure to the fibers thus forming a solid reinforcing structure. Upon application, and with the assistance of the air flow from the vacuum, the binder cools and solidifies into the desired preform shape. However, the application of the vacuum requires additional equipment in the form of a plenum arrangement and also requires additional control functions and labor to properly apply the fibers and vacuum. Therefore, the material and operating costs are increased.
In view of these prior art methods, it would be desirable to provide a simpler apparatus and a method for making preforms in which the problems associated with using solvent-borne, powdered or thermoplastic binders are minimized or overcome. It would also be desirable to provide apparatus and a method in which sagging, slumping, and separating of perform materials from tall vertical or nearly vertical surfaces is avoided. It would also be desirable to provide a lower cost method that is simple to operate and thus more conducive to automation. In a more simple forming process, it may even be possible to eliminate the need to transfer the preform to a molding tool and/or eliminate the need to apply a vacuum to the forming surface.